We examined butterfly species diversity on five of the Urato Islands, Japan (Fig. 1). Throughout the sampling period, a total of 39 species were identified and used in the study (see Materials and Methods: Mahanashi, 30; Hoh, 25; Katsura, 31; Nono, 34; Sabusawa, 35); these butterflies potentially use 58 host plant species [supporting information (SI) Fig. 3]. Of the butterfly species, nine are restricted to the use of a single host plant species, and, for eight species, the host plants are used by no other butterfly species. In this study, plant biomass was estimated as the weight of the edible parts of the plants. An analysis of the diversity and abundance of host plants and butterfly species was conducted for eight communities, of which two communities were classified on Katsura Island and three were classified on Sabusawa Island (the analysis of five communities, where each community corresponded to a single island, was also conducted, and the results are provided in SI Table 3 and SI Fig. 5). This community division was based on the observation that Katsura Island and Sabusawa Island support different vegetation types (Fig. 1; see also Materials and Methods).
The number of butterfly species did not differ greatly among the communities (26–30 species), and there was no significant effect of host plant species richness on butterfly species richness (regression analysis, b = 0.0884, F1,6 = 1.151, P = 0.3247, R2 = 0.1615, SI Fig. 4a). Simple regression analysis indicated that the total host biomass densities significantly affect butterfly biomass densities (b = 0.01513, F1,6 = 31.95, P = 0.0013, R2 = 0.8419, SI Fig. 4b), but butterfly biomass densities were not observed to have any effect on butterfly species richness (b = –0.491, F1,6 = 1.142, P = 0.3264, R2 = 0.1599, SI Fig. 4c).
By using an extension of the Mantel test (23, 24), we examined whether the similarities of the relative abundance pattern of butterfly species between the communities (Butterfly matrix) are determined by the similarities of the relative abundance pattern of their host plant species (Resource matrix), by geographic distances (Distance matrix), or by both. There was a clear correlation between the matrices of Butterfly and Resource, but there was no significant correlation between the matrices of Resource and Distance (r = 0.3945, P = 0.103). The multiple regression of the Resource and Distance matrices on Butterfly matrix indicated that the relative abundance pattern of butterflies was highly significantly influenced by the relative abundance pattern of their host plants, but not by geographic distance (Table 1 and Fig. 2). It is possible that communities within islands were similar in the relative abundance of butterflies. When within- and between-islands factors were used as independent variables instead of the Distance matrix for the multiple regression, the Butterfly matrix was not significantly influenced by within-island effects (P = 0.3996).
The effects of the biomass densities of host plant species on the density of each butterfly species that used them were examined. The results showed that in 23 (
60%) of 39 butterfly species, the regression model that included the biomass of one or more host plant species as explanatory variables showed a lower Akaike information criterion (AIC) value than that obtained without the biomass, and the densities positively increased with increasing host plant biomass (Table 2). After controlling the error of multiple comparisons, one or more host plant species weakly (8 butterfly species, P species, P
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